Computer simulation of the rheology of grafted chains under shear .2. Depletion of chains at the wall

We report nonequilibrium molecular dynamics simulations of grafted and free chains surrounded by solvent molecules and sheared between two atomic walls. Each wall is covered by a layer of amphiphilic molecules, which are twenty units long and chemically bounded to the surface. Simulations were performed at surface coverages ranging from 1/3 to 0, where 1/3 corresponds to a system with only grafted chains. Coverages lower than 1/3 were obtained by randomly detaching chains from the wall. The particles interact through the Weeks-Chandler-Andersen repulsive potential. Bond interactions and the stiffness of the chains are modeled using harmonic potentials. Heat is removed from the system through the walls by applying a Nose-Hoover thermostat. At coverages larger than 0, the chains behave like a wall resulting in steep velocity gradients. With decreasing coverages, the tilt of the amphiphiles is increased, and at the same time solvent molecules diffuse into the chain region. This effect of solvent molecules entering into the chain region is most pronounced at a coverage of 0.22. Frictional forces are higher for the intermediate coverages. This is probably due to entanglements between free and grafted chains. Decreasing the flexibility of the amphiphilic molecules creates a more dramatic response to the imposed shear field resulting in a larger chain tilt, higher frictional forces, and a higher solvent density at the wall.